Carbon sink#Natural sinks

In the context of climate change and in particular mitigation, a sink is defined as "Any process, activity or mechanism which removes a greenhouse gas, an aerosol or a precursor of a greenhouse gas from the atmosphere".IPCC, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_AnnexVII.pdf Annex VII: Glossary] [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C.  Méndez, S. Semenov, A. Reisinger (eds.)]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022.{{rp|2249}}

In the case of non-{{CO2}} greenhouse gases, sinks need not store the gas. Instead they can break it down into substances that have a reduced effect on global warming. For example, nitrous oxide can be reduced to harmless N₂.{{cite journal | vauthors = CHAPUIS-LARDY L, WRAGE N, CHOTTE J, BERNOUX M | title =Soils, a sink for N2O? A review | journal = Global Change Biology | volume = 13| pages =1–17 | date = 2007 | issue =1 | doi =10.1111/j.1365-2486.2006.01280.x | bibcode =2007GCBio..13....1C | s2cid =86551302 }}{{cite journal | vauthors = Cobo S, Negri V, Valente A, Reiner D, Hamelin L, Dowell N, Guillén-Gosálbez G | title = Sustainable scale-up of negative emissions technologies and practices: where to focus | journal = Environmental Research Letters | year = 2023 | volume =18 | issue = 2 | page = 023001 | doi = 10.1088/1748-9326/acacb3 | bibcode = 2023ERL....18b3001C | s2cid = 254915878 | hdl = 20.500.11850/596686 | hdl-access = free }}

Related terms are "carbon pool, reservoir, sequestration, source and uptake".{{rp|2249}} The same publication defines carbon pool as "a  reservoir in the Earth system where elements, such as carbon [...], reside in various chemical forms for a period of time."{{rp|2244}}

Both carbon pools and carbon sinks are important concepts in understanding the carbon cycle, but they refer to slightly different things. A carbon pool can be thought of as the overarching term, and carbon sink is then a particular type of carbon pool:{{Cite web |title=What is a Carbon Sink? |url=https://greenly.earth/en-us/blog/company-guide/what-is-a-carbon-sink |access-date=2024-11-23 |website=greenly.earth |language=en-us}} A carbon pool is all the places where carbon can be stored (for example the atmosphere, oceans, soil, plants, and fossil fuels).{{rp|2244}}

The amount of carbon dioxide varies naturally in a dynamic equilibrium with photosynthesis of land plants. The natural carbon sinks are:

• Soil is a carbon store and active carbon sink.{{cite journal |author=Blakemore, R.J. |title=Non-Flat Earth Recalibrated for Terrain and Topsoil |journal=Soil Systems |volume=2 |issue=4 |pages=64 |year=2018 |doi=10.3390/soilsystems2040064 |doi-access=free |bibcode=2018SoiSy...2...64B }}
• Photosynthesis by terrestrial plants with grass and trees allows them to serve as carbon sinks during growing seasons.
• Absorption of carbon dioxide by the oceans via solubility and biological pumps.

Artificial carbon sinks are those that store carbon in building materials or deep underground (geologic carbon sequestration).{{Cite journal |last1=Churkina |first1=Galina |last2=Organschi |first2=Alan |last3=Reyer |first3=Christopher P. O. |last4=Ruff |first4=Andrew |last5=Vinke |first5=Kira |last6=Liu |first6=Zhu |last7=Reck |first7=Barbara K. |last8=Graedel |first8=T. E. |last9=Schellnhuber |first9=Hans Joachim |date=2020 |title=Buildings as a global carbon sink |url=https://www.nature.com/articles/s41893-019-0462-4 |journal=Nature Sustainability|volume=3 |issue=4 |pages=269–276 |bibcode=2020NatSu...3..269C |doi=10.1038/s41893-019-0462-4 |issn=2398-9629 |s2cid=213032074}}{{Cite web |title=carbon sequestration {{!}} Definition, Methods, & Climate Change |url=https://www.britannica.com/technology/carbon-sequestration |access-date=2021-06-18 |website=Encyclopædia Britannica}} No major artificial systems remove carbon from the atmosphere on a large scale yet.{{Cite web|title=Carbon Sinks: A Brief Review|url=https://earth.org/data_visualization/carbon-sinks-a-brief-overview/|access-date=2020-12-02|website=Earth.Org - Past {{!}} Present {{!}} Future}}

Public awareness of the significance of {{CO2}} sinks has grown since passage of the 1997 Kyoto Protocol, which promotes their use as a form of carbon offset.{{Cite web |title=carbon sink – European Environment Agency |url=https://www.eea.europa.eu/help/glossary/eea-glossary/carbon-sink |access-date=2021-06-18 |website=eea.europa.eu}}

File:Carbon cycle.jpg shows the movement of carbon between land, atmosphere, soil and oceans in billions of tons of carbon per year. Yellow numbers are natural fluxes, red are human contributions in billions of tons of carbon per year. White numbers indicate stored carbon.]]{{Main|Carbon sequestration}}

Soils represent a short to long-term carbon storage medium and contain more carbon than all terrestrial vegetation and the atmosphere combined.{{cite journal |journal=Soil Science |volume=166 |issue=11 |pages=858–71 |last=Swift |first=Roger S. |title=Sequestration of Carbon by soil |date=November 2001 |doi=10.1097/00010694-200111000-00010 |bibcode=2001SoilS.166..858S |s2cid=96820247 }}{{Cite journal |last=Batjes |first=N.H. |date=1996 |title=Total carbon and nitrogen in the soils of the world |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1365-2389.1996.tb01386.x |journal=European Journal of Soil Science|volume=47 |issue=2 |pages=151–163 |doi=10.1111/j.1365-2389.1996.tb01386.x |bibcode=1996EuJSS..47..151B |issn=1351-0754}}{{Cite journal |last=Batjes |first=N.H. |date=2016 |title=Harmonized soil property values for broad-scale modelling (WISE30sec) with estimates of global soil carbon stocks |url=https://linkinghub.elsevier.com/retrieve/pii/S0016706116300349 |journal=Geoderma|volume=269 |pages=61–68 |doi=10.1016/j.geoderma.2016.01.034|bibcode=2016Geode.269...61B }} Plant litter and other biomass including charcoal accumulates as organic matter in soils, and is degraded by chemical weathering and biological degradation. More recalcitrant organic carbon polymers such as cellulose, hemi-cellulose, lignin, aliphatic compounds, waxes and terpenoids are collectively retained as humus.{{cite journal |author1=Klaus Lorenza |author2=Rattan Lala |author3=Caroline M. Prestonb |author4=Klaas G.J. Nieropc |title=Strengthening the soil organic carbon pool by increasing contributions from recalcitrant aliphatic bio(macro)molecules |journal=Geoderma |volume=142 |issue=1–2 |pages=1–10 |date=15 November 2007 |doi=10.1016/j.geoderma.2007.07.013 |url=https://zenodo.org/record/896885 |bibcode=2007Geode.142....1L }}

Organic matter tends to accumulate in litter and soils of colder regions such as the boreal forests of North America and the Taiga of Russia. Leaf litter and humus are rapidly oxidized and poorly retained in sub-tropical and tropical climate conditions due to high temperatures and extensive leaching by rainfall. Areas, where shifting cultivation or slash and burn agriculture are practiced, are generally only fertile for two to three years before they are abandoned. These tropical jungles are similar to coral reefs in that they are highly efficient at conserving and circulating necessary nutrients, which explains their lushness in a nutrient desert.{{cite web | access-date=19 September 2021 | title=Coral Reefs Biome "Underwater Rainforests" | url=https://untamedscience.com/biology/biomes/coral-reefs-biome/}}

Grasslands contribute to soil organic matter, stored mainly in their extensive fibrous root mats. Due in part to the climatic conditions of these regions (e.g., cooler temperatures and semi-arid to arid conditions), these soils can accumulate significant quantities of organic matter. This can vary based on rainfall, the length of the winter season, and the frequency of naturally occurring lightning-induced grass-fires. While these fires release carbon dioxide, they improve the quality of the grasslands overall, in turn increasing the amount of carbon retained in the humic material. They also deposit carbon directly into the soil in the form of biochar that does not significantly degrade back to carbon dioxide.{{Cite journal|last1=Woolf|first1=Dominic|last2=Amonette|first2=James E.|last3=Street-Perrott|first3=F. Alayne|last4=Lehmann|first4=Johannes|last5=Joseph|first5=Stephen|date=2010-08-10|title=Sustainable biochar to mitigate global climate change|journal=Nature Communications|volume=1|issue=5|pages=56|doi=10.1038/ncomms1053|issn=2041-1723|pmc=2964457|pmid=20975722|bibcode=2010NatCo...1...56W}}

Much organic carbon retained in many agricultural areas worldwide has been severely depleted due to intensive farming practices.{{cite web |date=17 October 2009 |title=Organic Farming Can Cool the World that Chemical Farming Overheated |url=https://www.cornucopia.org/2009/10/organic-farming-can-cool-the-world-that-chemical-farming-overheated/ |access-date=18 September 2021}} Since the 1850s, a large proportion of the world's grasslands have been tilled and converted to croplands, allowing the rapid oxidation of large quantities of soil organic carbon. Methods that significantly enhance carbon sequestration in soil are called carbon farming. They include for example no-till farming, residue mulching, cover cropping, and crop rotation.

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To enhance carbon sequestration processes in oceans the following technologies have been proposed but none have achieved large scale application so far: Seaweed farming, ocean fertilisation, artificial upwelling, basalt storage, mineralization and deep sea sediments, adding bases to neutralize acids. The idea of direct deep-sea carbon dioxide injection has been abandoned.{{Cite journal |last1=Benson |first1=S.M. |last2=Surles |first2=T. |date=2006-10-01 |title=Carbon Dioxide Capture and Storage: An Overview With Emphasis on Capture and Storage in Deep Geological Formations |url=https://zenodo.org/record/1232299 |url-status=live |journal=Proceedings of the IEEE |volume=94 |issue=10 |pages=1795–1805 |doi=10.1109/JPROC.2006.883718 |issn=0018-9219 |archive-url=https://web.archive.org/web/20200611063420/https://zenodo.org/record/1232299 |archive-date=11 June 2020 |access-date=10 September 2019 |s2cid=27994746}}

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{{See also|Green building and wood}}

File:Mjøstårnet.jpg, one of the tallest timber buildings, at its opening 2019]]

Broad-base adoption of mass timber and their role in substituting steel and concrete in new mid-rise construction projects over the next few decades has the potential to turn timber buildings into carbon sinks, as they store the carbon dioxide taken up from the air by trees that are harvested and used as mass timber. This could result in storing between 10 million tons of carbon per year in the lowest scenario and close to 700 million tons in the highest scenario. For this to happen, the harvested forests would need to be sustainably managed and wood from demolished timber buildings would need to be reused or preserved on land in various forms.

Using rapidly renewable plant materials like bamboo, straw or hempcrete can be further carbon sinks.{{Cite journal |last=Jami |first=Tarun |last2=Karade |first2=S. R. |last3=Singh |first3=L. P. |date=2019-12-01 |title=A review of the properties of hemp concrete for green building applications |url=https://www.sciencedirect.com/science/article/abs/pii/S0959652619327222?via=ihub |journal=Journal of Cleaner Production |volume=239 |pages=117852 |doi=10.1016/j.jclepro.2019.117852 |issn=0959-6526}}{{Cite journal |last=Arrigoni |first=Alessandro |last2=Pelosato |first2=Renato |last3=Melià |first3=Paco |last4=Ruggieri |first4=Gianluca |last5=Sabbadini |first5=Sergio |last6=Dotelli |first6=Giovanni |date=2017-04-15 |title=Life cycle assessment of natural building materials: the role of carbonation, mixture components and transport in the environmental impacts of hempcrete blocks |url=https://www.sciencedirect.com/science/article/abs/pii/S0959652617303876 |journal=Journal of Cleaner Production |volume=149 |pages=1051–1061 |doi=10.1016/j.jclepro.2017.02.161 |issn=0959-6526|hdl=10446/131799 |hdl-access=free }}

{{Portal|Climate change|Energy|Engineering|Environment}}

• Carbon budget
• Forest management
• Reforestation

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